1. Field of the Invention
The present invention relates to a stage apparatus for positioning a workpiece or other object to be processed or machined. More particularly, the present invention relates to a stage apparatus suitable for use in an exposure system.
2. Description of the Related Art
In an exposure system used to produce, for example, semiconductor devices, liquid crystal display devices, image pick-up devices (e.g. CCDs), or thin-film magnetic heads, a reticle stage or a wafer stage is used as a stage apparatus for positioning a reticle (or a photomask, etc.) or a wafer (or a glass plate, etc.). These days, exposure systems of the scanning exposure type, e.g., step-and-scan exposure systems, are also used in which a pattern on a reticle is sequentially transferred onto a wafer by synchronously scanning the reticle and the wafer relative to a projection optical system. Stage apparatuses used in such scanning exposure type systems are also demanded to have the function of scanning the reticle and the wafer at a constant speed and with high accuracy.
Reticle and wafer stages for an exposure system are each provided with a laser interferometer (i.e. laser light wave interference type length measuring device) for precisely measuring the position of the stage at the time of positioning or scanning. The laser interferometer measures the position of the stage by applying a laser beam to a moving mirror secured to the stage and receiving the laser beam reflected from the moving mirror. In this case, if there are fluctuations of air in the optical path of the laser beam, an error is introduced into the measured value by the laser interferometer. Such fluctuations of air are, in many cases, caused by a turbulent flow or air or a change in the environmental temperature in the vicinity of the optical path. Therefore, there has been a demand for a method of minimizing a turbulent flow of air and a temperature change in the vicinity of the optical path.
Recently, linear motors have been used in reticle and wafer stages as driving mechanism for driving the stages at high speed and in a non-contact fashion. A typical linear motor consists essentially of a stationary member secured to a base member and a moving member secured to a member that moves relative to the base member. When the stationary member includes a coil, the moving member includes a magnetic field-generating member, e.g. a magnet, whereas, when the stationary member includes a magnetic field-generating member, the moving member includes a coil. A linear motor of the type wherein a magnetic field-generating member is included in the moving member and a coil is included in the stationary side is called xe2x80x9cmoving magnet type linear motorxe2x80x9d. A linear motor of the type wherein a coil is included in the moving member and a magnetic field-generating member is included in the stationary member is called xe2x80x9cmoving coil type linear motorxe2x80x9d.
Both the moving magnet type linear motor and the moving coil type linear motor structurally need a wide gap between the coil and the magnetic field-generating member in comparison to the ordinary rotary motors and therefore tend to exhibit a lower efficiency and to generate a large amount of heat than the ordinary rotary motors.
In many cases, driving coils used in linear motors according to the foregoing prior art have a structure in which the coils are exposed in the air. Accordingly, heat from the coils causes a change in the ambient air temperature. This causes fluctuations of air around the optical path of a laser beam emitted from a laser interferometer that measures the position of the reticle or wafer stage, resulting in errors in the measured values by the laser interferometer. However, the errors have heretofore been within a specified tolerance in most case. These days, however, the integration degree of semiconductor devices and the like is increasing more and more, and a correspondingly high positioning accuracy is required for exposure systems. Therefore, it is demanded to reduce measuring errors due to local fluctuations of air and other disturbance.
Accordingly, there has recently been proposed a method wherein the area around coils that generate a large amount of heat is covered with a cylindrical container, and a cooling fluid is passed through the container by a temperature controller to prevent a rise in temperature which would otherwise be caused by heat generated from the coils. This method makes it possible to suppress a rise in temperature of the heat-generating portion. However, according to this method, a cooling fluid is simply passed through the heat-generating portion. That is, a cooling fluid is passed in disregard of the position of a laser interferometer, or in a case where a plurality of linear motors are installed, the linear motors are cooled successively by a cooling fluid through a single system of circulating piping. Therefore, the conventional method cannot attain the object to eliminate a change in the ambient temperature of the optical path of the laser beam from the laser interferometer to thereby surely control local fluctuations of air around the optical path.
In view of the above-described circumstances, a first object of the present invention is to provide a stage apparatus that uses a linear motor to drive a movable portion and that measures the position of the movable portion with an interferometer. The stage apparatus is designed to suppress fluctuations of air in the optical path of a light beam emitted from the interferometer due to heat generated from the linear motor, thereby enabling the position of a stage to be measured with high accuracy, and thus allowing the stage to be accurately positioned or moved.
In an exposure system, e.g. stepper, a wafer stage, for example, is used as a device for moving a wafer to a predetermined exposure position. The wafer stage includes an X-axis stage and a Y-axis stage, which are movable in respective directions parallel to X- and Y-axes perpendicularly intersecting each other. These days, particularly, a hydrostatic air guide type stage apparatus that uses a hydrostatic gas bearing is used as a wafer stage for realizing high-speed and high-accuracy positioning. Regarding exposure systems, attention has also been paid to scanning exposure type projection exposure systems such as step-and-scan type exposure systems, in which exposure is carried out by synchronously scanning a reticle and a wafer. In such a scanning exposure type projection exposure system, a hydrostatic air guide type stage apparatus is used not only for a wafer stage but also for a reticle stage. Further, hydrostatic air guide type stage apparatuses are also used for positioning workpieces or other objects to be processed or machined, for example, in precision machine tools or precision measuring machines.
A conventional hydrostatic gas bearing used in such a stage apparatus is schemed to maintain a constant gap between a movable portion and a stationary portion by keeping a constant static pressure between the moveable portion and the stationary portion, thereby enabling the movable portion to be moved smoothly at high speed. The conventional hydrostatic gas bearing has a hydrostatic gas outlet and inlet provided in the bearing surface of the movable portion or the stationary portion (there are cases where no inlet is provided). Thus, a constant gap is maintained between the bearing surface and a guide surface facing the bearing surface by the balance of repulsion force produced by blowoff of compressed air supplied from the outside and suction force (or gravity).
In an exposure system, laser interferometers are used to measure the positions of a reticle stage and wafer stage. A laser interferometer applies a laser beam to a moving mirror secured to a movable portion to measure an amount of displacement of the moveable portion. In this case, if there are fluctuations of air in the optical path of the laser beam, an error is introduced into the measured value by the laser interferometer. Therefore, measures have heretofore been taken to suppress the fluctuations of air in the optical path of the laser beam. For example, temperature-controlled air is supplied to the surroundings of the optical path of the laser beam.
A conventional stage apparatus using a hydrostatic gas bearing has a structure in which the compressed air blown is constantly released as it is to the periphery of the air outlet provided in the bearing surface. However, in general factories, the temperature of compressed air or the like that is supplied to the hydrostatic gas bearing is not satisfactorily controlled. Even if the compressed air is at room temperatures, when it is blown out from the air outlet provided in the bearing surface, the air pressure reduces to the atmospheric pressure, and the temperature of the air is undesirably lowered by adiabatic cooling. Accordingly, the ambient temperature of the workpiece lowers below the desired temperature. Consequently, it is likely that a positioning error or a machining error will occur on account of the contraction of the workpiece, the stage, etc. As has been stated above, may of stages for exposure systems use laser interfereometers that use laser beams to measure coordinate positions. Accordingly, it is necessary in order to perform accurate coordinate measurement to suppress a turbulent flow of air in the optical path of a laser beam emitted from such a laser interferometer and a temperature change in the optical path. However, if air different in temperature from the environmental air is discharged from a hydrostatic gas bearing, the air stream in the optical path of the laser beam is made turbulent. This causes temperature variations in the optical path, resulting in a degradation of the measuring accuracy of the laser interferometer.
Compressed air to be supplied to a hydrostatic gas bearing is not satisfactorily cleaned in general factories. Even air that is cleaned to a high degree cannot avoid mixing of fine foreign substances. Therefore, the workpiece may be contaminated by fined foreign substances or chemical substances released into the air from the hydrostatic gas bearing. It is necessary to prevent missing of such foreign substances particularly in factories where a high level of cleanliness is required to produce semiconductor substrates, liquid crystal substrates, etc.
In view of the above-described circumstances, an object of the present invention is to provide a stage apparatus designed so that when a hydrostatic gas bearing is used, the amount of air released irregularly to the surroundings of a bearing surface is minimized to suppress contamination on a stage and to minimize a change in the environmental temperature. A second object of the present invention is to provide a stage apparatus designed so that when an interferometer is used in combination with a hydrostatic gas bearing, the measuring accuracy of the interferometer is improved to accurately measure the position of a stage, thereby enabling the stage to be positioned or moved with high accuracy.
A stage apparatus according to the present invention includes a movable member; a linear motor that drives the movable member in a predetermined direction; an interferometer that applies a light beam to the movable member to measure the position of the movable member in the predetermined direction; and a fluid supply device that supplies a temperature-controlled fluid to the linear motor from a side thereof closer to the optical path of the light beam from the interferometer such that the fluid flows around a driving coil of the linear motor.
By virtue of the above-described arrangement of the stage apparatus according to the present invention, a temperature-controlled fluid is supplied to the linear motor from a side thereof closer to the optical path of the light beam emitted from the interferometer. Accordingly, the gas temperature at a portion of the linear motor in the vicinity of the optical path of the light beam can be readily controlled to a desired temperature of the optical path of the light beam by using the fluid before the temperature thereof is raised by the driving coil. For example, if the temperature of the fluid to be supplied to the linear motor is set to a level close to the desired temperature of the optical path of the light beam, the temperature of the linear motor in the vicinity of the optical path of the light beam is maintained substantially at the desired temperature, and thus fluctuations of air in the optical path of the light beam from the interferometer are favorably suppressed. Accordingly, the position of the movable member can be measured with high accuracy by the interferometer, and the movable member can be positioned accurately.
By absorption of heat from the driving coil, the temperature of the fluid rises, and a temperature difference is produced between the temperature of the fluid at a fluid inlet of the linear motor and the temperature of the fluid at a fluid outlet of the linear motor. As a result, a temperature gradient is produced in the longitudinal direction of the linear motor. In this case, however, a gas stream induced by the temperature gradient flows from an end of the linear motor on a side thereof closer to the optical path of the light beam from the interferometer toward the other end of the linear motor on the opposite side. Usually, the whole stage apparatus is placed in an overall air-conditioned environment, and the system is set so that a gas stream produced by the overall air conditioning (the gas stream will be hereinafter referred to as xe2x80x9cenvironmental gas streamxe2x80x9d) flows from a side of the system closer to the optical path of the light beam from the interferometer toward the stage apparatus. Accordingly, the flow direction of the gas stream induced by the temperature gradient on the surface of the linear motor is coincident with the flow direction of the environmental gas stream. Therefore, no turbulence will occur in the gas stream.
The stage apparatus may have a plurality of linear motors to drive the movable member in the predetermined direction, and the fluid supply device may be arranged to supply the temperature-controlled fluid to the linear motors in parallel from a side of each linear motor closer to the optical path of the light beam emitted from the interferometer. By doing so, a temperature gradient similar to the above occurs in the longitudinal direction of each of the linear motors. Because the temperature gradient is such that the temperature becomes higher as the distance from the optical path of the light beam increases toward an area where the light beam from the interferometer does not pass, a gas stream induced by the temperature gradient is an orderly stream flowing in the same direction as the environmental gas stream. Therefore, it is possible to suppress fluctuations of air in the optical path of the light beam.
The linear motor may be a moving magnet type linear motor. In this case, the driving coil is incorporated in the stationary member. Therefore, it becomes easy to route piping for a fluid supplied to cool the driving coil.
A stage control method according to the present invention includes the step of driving a movable member in a predetermined direction by a linear motor, and while doing so, applying a light beam to the movable member from an interferometer to measure the position of the movable member in the predetermined direction, and the step of supplying a temperature-controlled fluid to the linear motor from a side thereof closer to the optical path of the light beam from the interferometer such that the fluid flows around a driving coil of the linear motor.
The stage control method according to the present invention may include the step of driving the movable member in the predetermined direction by a plurality of linear motors, and the step of supplying the temperature-controlled fluid to the linear motors in parallel from a side of each linear motor closer to the optical path of the light beam from the interferometer.
A stage apparatus according to another aspect of the present invention includes a first member having a guide surface; a second member having a bearing surface facing the guide surface; a hydrostatic gas bearing that blows compressed air over the guide surface from a gas outlet provided in the bearing surface, so that the first member and the second member are moved relative to each other through the hydrostatic gas bearing; and an exhaust groove for discharging a gas which is provided around the gas outlet provided in the bearing surface.
By virtue of the above-described arrangement of the stage apparatus according to the present invention, an exhaust groove for discharging a gas is provided around the gas outlet. Therefore, the gas blown out of the gas outlet is discharged in a desired direction through the exhaust groove. Accordingly, the amount of gas irregularly released directly to the surroundings of the bearing surface reduces. Therefore, even when a hydrostatic gas bearing is used, contamination of the stage apparatus by the discharged gas is suppressed, and a change in the environmental temperature is minimized.
In this case, the bearing surface may be provided with partition walls flush with the bearing surface such that the partition walls face each other across the exhaust groove. With this arrangement, the amount of gas released irregularly from a gap between the guide surface of the first member and the bearing surface further reduces.
In a case where a temperature-controlled gas stream is supplied to the stage apparatus in a predetermined direction, the discharge opening of the exhaust groove may be provided on the leeward of the temperature-controlled gas stream. By virtue of this arrangement, it is possible to suppress the turbulence of gas stream and temperature change caused by the gas discharged from the exhaust groove.
It is also possible to provide a temperature sensor that measures the temperature of the gas discharged from the exhaust groove and to control the temperature of a compressed gas source for the hydrostatic gas bearing on the basis of the value of temperature measured by the temperature sensor such that the temperature of the gas discharged from the exhaust groove becomes equal to the temperature of the temperature-controlled gas stream. By doing so, the exhaust gas is discharged at a temperature equal to that of the temperature-controlled gas stream. Accordingly, a temperature change of the gas stream can be further reduced.
The arrangement may be such that the stage apparatus is further provided with an interferometer that applies a light beam to at least one of the first and second members to detect a relative displacement between the two members, and that the gas discharged from the exhaust groove is released to a position away from the optical path of the light beam from the interferometer. By virtue of this arrangement, fluctuations of air in the optical path of the light beam from the interferometer reduce, and thus measuring errors of the interferometer reduce. Accordingly, the first and second members can be positioned relative to each other with high accuracy.
A stage control method according to another aspect of the present invention includes the step of moving a first member having a guide surface and a second member having a bearing surface facing the guide surface relative to each other while blowing compressed air over the guide surface from gas outlet provided in the bearing surface between the first member and the second member, and the step of discharging a gas from an exhaust groove provided around the gas outlet in the bearing surface during the above step of moving the first and second members relative to each other.
The stage control method may further include the step of supplying a temperature-controlled gas stream to the first and second members in a predetermined direction, and the step of discharging a gas from the exhaust groove on the leeward of the temperature-controlled gas stream.
The stage control method may further include the step of measuring the temperature of the gas discharged from the exhaust groove, and the step of controlling the temperature of a compressed gas source on the basis of the measured temperature such that the temperature of the gas discharged from the exhaust groove becomes equal to the temperature of the temperature-controlled gas stream.
The stage control method may further include the step of applying a light beam from an interferometer to at least one of the first and second members to detect a relative displacement between the two members, and the step of releasing the gas discharged from the exhaust groove to a position away from the optical path of the light beam from the interferometer.